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Thu, 31 Jul 2025 11:16:31 -0700 (PDT) Date: Thu, 31 Jul 2025 14:16:30 -0400 From: Johannes Weiner To: SeongJae Park Cc: Andrew Morton , Chengming Zhou , Nhat Pham , Takero Funaki , Yosry Ahmed , kernel-team@meta.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC PATCH] mm/zswap: store compression failed page as-is Message-ID: <20250731181630.GB1055539@cmpxchg.org> References: <20250731152701.GA1055539@cmpxchg.org> <20250731170922.15509-1-sj@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250731170922.15509-1-sj@kernel.org> On Thu, Jul 31, 2025 at 10:09:22AM -0700, SeongJae Park wrote: > On Thu, 31 Jul 2025 11:27:01 -0400 Johannes Weiner wrote: > > So if compression fails, still do zpool_malloc(), but for PAGE_SIZE > > and copy over the uncompressed page contents. > > > > struct zswap_entry has a hole after bool referenced, so you can add a > > flag to mark those uncompressed entries at no extra cost. > > > > Then you can detect this case in zswap_decompress() and handle the > > uncompressed copy into @folio accordingly. > > I think we could still use 'zswap_entry->length == PAGE_SIZE' as the indicator, > As long as we ensure that always means the content is incompressed, following > Nhat's suggestion[1]. > > Please let me know if I'm missing something. Ah, right. So if compression succeeds but the result is still PAGE_SIZE you will treat it as failure and store it uncompressed. Then PAGE_SIZE always means uncompressed for the readers. Even better.